Muzzle and Nose
A number of things make a dog's scenting ability superior. The shape and length of a dog's muzzle is uniquely suited to extreme smelling ability. Starting at the nostrils, their shape is unique to allow expiration out of the sides of the nose so that fewer scent molecules on the ground or in the air are disturbed.
A dog's nose is normally cool and moist. The moisture secreted by mucus glands in the nasal cavity captures and dissolves molecules in the air and brings them into contact with the specialized olfactory epithelium.